The present invention relates in general to the field of sensor technique and in particular to capacitive pressure or force sensors for measuring physical quantities such as force, absolute pressure, differential pressure, deflection, spacing, and the like.
With capacititve sensors, such as described for example in European patent 0 461 459, the path of the characteristic curve, i.e. the pressure sensor output signal is determined as a function of the quantity to be measured, in essence by the mechanical properties thereof. This means that the deformation of a deflectable membrane above a basic structure or the displacement of a deflecting beam as a result of an external force directly determines the path of the characteristic curve if a sheet-like electrode on the movable element is located opposite a sheet-like electrode on the fixed element.
WO 90/12299 describes a capacitive pressure sensor in which the deflectable element, i.e. the membrane, has a single sheet-like electrode whereas the basic structure has already three electrodes provided thereon, one thereof delivering a constant output signal irrespective of the pressure applied, while the two other electrodes, together with the electrode attached to the membrane, each deliver two capacitive output signals which, with the same force applied, are different.
Disadvantageous in this arrangement is on the one hand the relatively complicated shape of the individual electrodes. The electrode configuration, moreover, cannot be described directly by geometric data, such as e.g. angles and associated radii. On the other hand, the electrode areas both of the membrane and of the supporting structure must be designed anew for different applications. Moreover, in the arrangement described it is necessary that all electrodes of the supporting structure as well as all electrodes of the membrane must be contacted separately in order to make sure that the indivdidual measuring capacitances are at different potentials, so that a fedback evaluation circuit can be used that is based on the switched-capacitor principle.
WO 93/11415 discloses a capacitive pressure measurement arrangement with high linearity. The pressure measurement arrangement comprises first and second measuring capacitances as well as two pressure-dependent capacitances. The electrode areas for defining the measuring capacitances and the pressure-dependent capacitances suitably designed for obtaining a linear output signal. The pressure sensor structure comprises furthermore two reference capacitances, with all capacitances being connected to each other by means of an external circuit arrangement such that the two measuring capacitances and the two reference capacitances are connected to each other in a bridge circuit, whereas the two additional pressure-dependent capacitances are each located in the feedback branches of a differential amplifier. The counter-structure has the same electrode area configuration as the membrane structure so as to render possible that all capacitances have different potentials applied thereto. Every electrode area can be realized by individual partial electrode areas or by a continuous electrode area.
WO 95/03534 discloses capacitive pressure sensors with high linearity. A chamber-side area of a membrane for pressure measurement is covered with a reference electrode and a measuring electrode. The counter-structure has a continuous electrode so that individual capacitances are formed between the electrode structures and the electrode covering the entire area, with said capacitances each having a terminal with the same potential.
U.S. Pat. No. 4,977,480 discloses a capacitance-type sensor, in which a capacitor with constant capacitance is connected in series with a capacitor with variable capacitance. The capacitance of the variable-capacitance capacitor varies in accordance with a pressure applied to the sensor. The variable-capacitance capacitor comprises a movable electrode on a membrane and a fixed electrode disposed opposite the movable electrode. The fixed counter-electrode furthermore is structured so as to form an additional counter-electrode for establishing a further variable capacitance that is connected in parallel to the other capacitance. A further variable-capacitance capacitor is connected in series with a constant-capacitance capacitor. All capacitances comprise an electrode on the membrane and another electrode on the counter-structure or, respectively, a capacitance remote from the measuring chamber in case of the fixed capacitances.
EP 0 195 985 discloses a capacitive pressure sensor comprising a common first electrode on a membrane as well as second and third electrodes, opposite the first electrode, on a counter-structure. Contact is established both with the electrodes on the counter-structure and with the electrode on the membrane, so as to establish two pressure-sensitive capacitors, with the change of capacitance thereof as a function of the pressure being the same for each.
DE-U-9013959 relates to a sensor for a capacitance manometer. The manometer comprises a base disc and a membrane connected to the same. On the inside of the base disc, there are provided at least two separate metallic layer sections which, together with electrically interconnected metallic layer sections on the membrane, constitute two series-connected capacitances the terminals of which are provided on the base disc. In case of this sensor it is no longer necessary to equip the membrane with electric contacts.
It is the object of the present invention to create a concept that provides for a simple pressure or force sensor structure for use in flexible manner, in which the path of the characteristic curve thereof can be brought into conformity with a desired path of the characteristic curve.
This object is met by a pressure or force sensor structure according to claim 1 as well as by a method for producing a pressure or force sensor structure according to claim 15.
A force or pressure sensor structure according to the invention comprises a membrane and a counter-structure, with the membrane and the counter-structure each having electrodes provided thereon for determining capacitors. For determining a desired pressure/capacitance dependence or force/capacitance dependence, respectively, at least two of the capacitors are connected in series or in parallel.
The present invention is based on the finding that, for obtaining independence of the path of the characteristic curve, there is provided a plurality of individual capacitors by corresponding electrode configuration in a pressure or force sensor structure, which with the same deflection deliver different capacitances. By connecting these capacitors in parallel or in series, it is possible then to obtain a combined capacitance curve as a function of the deflection of the membrane, which corresponds to a desired path. This means thus that the desired capacitance curve is obtained on the one hand by a geometric configuration of the capacitor electrodes in the capacitive sensor and on the other hand by interconnecting at least two capacitors having a corresponding configuration. Thus, there are at least two degrees of freedom present for obtaining a desired dependence.
Preferably, a large number of capacitors is determined by a large number of correspondingly shaped electrodes which then are suitably connected by means of an evaluation circuit with feedback feature whereby very complicated, and in particular also non-linear, desired pressure/capacitance dependences and force/capacitance dependences, respectively, can be simulated as well.
An advantage of the present invention furthermore consists in that it can be used for various applications, i.e. various dependences, without substantial modification. To this end, either the membrane or the counter-structure and preferably the counter-structure is provided with a multiplicity of circle segment-shaped electrode areas in the shape of segments of a circle (e.g. when considering a membrane or a counter-structure with the shape of a circle), which together with circle segment-shaped areas of the membrane define capacitors, with the possibility, by connecting various capacitors in series or in parallel, of so to speak xe2x80x9cputting togetherxe2x80x9d or xe2x80x9cassemblingxe2x80x9d arbitrary desired electrode areas that were calculated in a preceding numerical simulation. In particular when a multilayer construction is used for the counter-structure, this provides an extremely flexible concept since the metallization structuring both of the membrane and of the counter-tructure as well as the evaluation circuit employed are the same for any application, i.e. for any pressure/capacitance dependence or force/capacitance dependence, respectively, whereas the parallel connection of individual capacitors, i.e. the xe2x80x9cassemblingxe2x80x9d of the calculated electrode areas, is effected only by multilayer-structuring of the counter-structure or also of the membrane. The resolution of the area, i.e. the shape of the areas that can be xe2x80x9cassembledxe2x80x9d is determined by the number of the individual electrode areas and opposed electrode areas of the other element of the pressure or force sensor structure. If a sufficient number of small electrode areas is provided, corresponding, numerically calculated electrode areas can be assembled in virtually arbitrary manner by connecting corresponding capacitor elements in parallel.
For reducing the thus increasing contacting expenditure, a concept is used according to the invention in which, differently from the prior art in which the electrodes both of the membrane and of the counter-structure have to be contacted so far, only either the membrane or the counter-structure is contacted. This contributes in providing the effect that the membrane and the counter-structure have the same metallization for any dependence desired, since the so to speak macroscopic design of the electrodes is effected just by interconnecting individual elementary capacitors, preferably using multilayer structuring on one element only.
Furthermore, one should not underestimate the fact that, for all dependences desired, it is possible to employ just one single evaluation circuit in the form of an electronic integrated circuit, that is the same for all desired dependences and thus can be produced mostly in large numbers of pieces and thus in inexpensive manner. This is particularly significant for capacitive pressure or force sensor structures since these are mostly employed in large numbers of pieces at many locations of a system. The multilayer structuring, in particular if it is applied to the fixed element of the pressure or force sensor structure or even constitutes said fixed element, can be provided using very inexpensive materials and, despite the different design for each dependence, can be established in very inexpensive manner in due to the high standard of sophistication in multilayer technology.